Clemens, in U.S. Pat. No. 3,842,194, discloses a capacitive video disc playback system which includes a high density information record having a conductive surface. Initially, information records for this system were sandwich-type constructions, e.g. the record was formed of a plastic disc coated with a layer of conductive metal which was in turn coated with a dielectric material, such as polystyrene. Subsequently improved information discs for this system were homogeneous and comprised a thermoplastic matrix having embedded therein sufficient finely divided conductive carbon black particles to obtain capacitive playback. Suitable molding compositions for such discs are disclosed in Martin et al U.S. Pat. No. 4,228,050.
Conductive molding compositions, such as described by Martin et al, are comparatively expensive due principally to the amount of high quality, low density, conductive carbon black contained therein. Additional various additives, such as stabilizers, lubricants and the like, in such compositions also contribute to the cost of a disc produced therefrom. Other workers have sought to reduce the cost of such a disc by bonding thin layers of the expensive conductive molding composition onto an inexpensive support or core disc to form a sandwich-type disc.
O'Mara, in U.S. Pat. No. 4,390,487, discloses a method of forming a sandwich-type capacitive electronic disc ("CED") into which the conductive molding composition is injected into a compression mold and a nonconductive core material is injected inside of the conductive plastic to form a preform which is compression molded to form a CED.
Ruda, in copending U.S. patent application Ser. No. 522,332, filed Aug. 11, 1983, discloses a sandwich-type CED having comparatively thin conductive layers, i.e. about 2 to 30 mils thick, which are formed by passing the conductive molding composition, in molten form, between rollers in production calendering equipment or in a two-roll mill. The conductive composition is, in essence, that disclosed by Martin et al.
Dixon et al, in application Ser. No. 556,354, filed Nov. 29, 1983, now abandoned, disclose preparation of a sandwich-type CED by forming a dispersion of conductive carbon black particles in a solution of a thermoplastic resin such as polyvinyl chloride, and certain additives, coating the dispersion onto a core disc and drying to form conductive layers which are then embossed with an information pattern. Di Marco, in application Ser. No. 556,355, filed Nov. 29, 1983, now U.S. Pat. No. 4,515,830, discloses forming a similar dispersion from the heat-treated conductive molding composition disclosed by Martin et al and preparing sandwich-type CED's therefrom in a similar manner. The conductive layers of Dixon et al and DiMarco are substantially thinner than those of Ruda.
The conductive layer in the CED's produced by each of the above methods contains conductive carbon black particles and additives for the processing and stabilization of the thermoplastic resin. In addition to the cost of the conductive carbon, the abrasiveness of molding compositions such as disclosed by Martin et al causes the stampers used to emboss the information pattern in the conductive layer or disc to wear out or score rather quickly and, thus, become unuseable. This adds another cost to the prouction of CED's. There is also to be considered in each instance possible reactions and/or degradations among the various components of the formulation during the heat and pressure of compression molding.
In accordance with this invention, there is provided a method of preparing sandwich-type CED's which have conductive layers at least as thin as those of Dixon et al and DiMarco. The conductive layers of the subject CED's are homogeneous and contain no conductive carbon black or other conductive particles.